1917] Stock: Structure of the Pes in Mylodon harlani 



283 



he states 30 is "quite similar to that of the Megalonychidae, but a 

 marked and suggestive likeness to Mylodon is seen in the trochlea; 

 the inner condyle has a very distinct tendency to assume the odontoid 

 shape and is much less extended proximo-distally than the external 

 condyle." It is worthy of note that Scott considers the astragalus 

 of Prepotherium as approaching that of Megatherium in the proximo- 

 distal shortness and strong convexity of the tibial articulating surface 

 and its prominent inward projection. Scott believes that in Prepo- 

 therium these characters indicate "a beginning had been made in the 

 rotation of the foot, so as to bring the weight upon the fibular border. ' ' 

 The pronounced development of the odontoid, inner division of 

 the tibial articulating surface of the astragalus in M. harlani, and the 

 distinct separation between it and the outer division in the Rancho 

 La Brea species indicate a greater specialization than in Megalonyx 

 and Nothrotherium. It represents the result of a differentiation which 

 had already commenced in the Miocene. The extreme modification 

 of the astragalus in Mylodon would seem to suggest that the weight 

 of the body was often supported by the posterior limbs alone. This 

 is in accord with Owen's conception of the form as a browsing type, 

 often resting its weight on the posterior extremities while using the 

 anterior limbs to bring branches and foliage of trees within reach of 

 the mouth. 



Allowing for the new interpretation of the second and third pha- 

 langes of digit 3, the pes of Mylodon harlani is close in stage of 

 evolution to that of M. robust us. In the distinct angle between the 

 divisions of the tibial articulating surface of the astragalus, M. harlani 

 is more advanced than the type of M. robustus as figured by Owen. 

 This character is, however, somewhat variable in the North American 

 species. A greater degree of advancement in M. harlani is suggested 

 by the anteroposterior shortening of metatarsal 2, and the greater 

 anteroposterior extension of the middle cuneiform than in M. robustus. 



The co-ossification of the first and second phalanges of the median 

 digit is characteristic of Megatherium, Megalonyx, Nothrotherium, 

 and, among the Mylodontidae, of Scelidotherium and Catonyx. Fusion 

 of the two phalanges has been noted in Mylodon garmani and may 

 occur in M. harlani, but is apparently absent in M. robustus. In 

 Lestodon and Pseudolestodon the two phalanges are separate. The 

 widespread occurrence of this co-ossification among Pleistocene Gravi- 



30 Scott, W. B., Eeports of the Princeton University Expedition to Patagonia, 

 1896-1899, vol. 5, pt. 1, Edentata, 3, Gravigrada, p. 350, 1904. 



